Organic electronic devices provide many potential advantages including inexpensive, low temperature, large scale fabrication on a variety of substrates including glass and plastic. Organic light emitting diode displays provide additional advantages as compared with other display technologies—in particular they are bright, colourful, fast-switching and provide a wide viewing angle. OLED devices (which here includes organometallic devices and devices including one or more phosphors) may be fabricated using either polymers or small molecules in a range of colours and in multicoloured displays depending upon the materials used. For general background information reference may be made, for example, to WO90/13148, WO95/06400, WO99/48160 and U.S. Pat. No. 4,539,570, as well as to “Organic Light Emitting Materials and Devices” edited by Zhigang Li and Hong Meng, CRC Press (2007), ISBN 10: 1-57444-574X, which describes a number of materials and devices, both small molecule and polymer.
In its most basic form an organic light emitting diode (OLED) comprises a light emitting layer which is positioned in between an anode and a cathode. Frequently a hole injection layer is incorporated in between the anode and the light emitting layer. It functions to decrease the energy difference between the work function of the anode and the highest occupied molecular orbital (HOMO) of the light emitting layer thereby increasing the number of holes introduced into the light emitting layer. In operation holes are injected through the anode, and if present the hole injection layer, into the light emitting layer and electrons are injected into the light emitting layer through the cathode. The holes and electrons combine in the light emitting layer to form an exciton which then undergoes radiative decay to provide light.
In many OLEDs the light emitting layer comprises a light emitting compound and a charge transporting material, e.g. a electron transporting polymer. A wide range of light emitting compounds are employed. These are generally metal complexes of lanthanide or d-block metals such as iridium. Typically 40-50% wt of the light emitting layer is light emitting compound and the remainder is charge transporting material. Such devices have excellent optoelectronic properties, namely high luminance, current density and EQE for a given drive voltage as well as luminance over extended periods of time (i.e. extended device lifetime).
The drawback of such devices, however, is that the light emitting compounds, present in amounts of up to 50% wt of the light emitting layer, are expensive and this significantly increases the overall cost of the manufacture of the devices. A need therefore exists for new devices that provide comparable optoelectronic properties to todays commercially available devices but which are less expensive to prepare.